Method of annealing nonferrous metal parts without stickers

ABSTRACT

Process to avoid adhesions during the annealing of non-ferrous metal alloys, comprising the heating, holding and cooling phases, whereby the material being annealed is exposed to an inert or oxidizing protective-gas atmosphere during the structure transformation, as a result of which a thin oxide layer is formed during this time on the surface of the material being annealed and/or an oxide layer that was previously there is maintained, thus preventing the non-ferrous metal objects from adhering together.

BACKGROUND OF INVENTION

The invention relates to a process to anneal non-ferrous metal objectsin an adhesion-free manner, that is to say, to avoid so-called adhesionsduring the annealing procedure, especially in a bell-type furnace.

Non-ferrous metals such as, for instance, bronze wires or bronze stripsundergo a homogenizing annealing step after the casting and shaping.Subsequently, additional shaping measures such as rolling or drawing aretaken and re-crystallization annealing steps are carried out.

The annealing temperatures lie between 300° C. and 700° C. [572° F. and1292° F.]. The annealing is conducted in continuous furnaces, arelatively complex approach in view of the fact that the objects usuallyhave a small cross section.

When annealing coils, which can be done, for example, in bell-typefurnaces, local diffusion welds, so-called adhesions, occur at thecontact sites of the objects, for example, between individual windingsof the wound-up wire or strip, due to diffusion mechanisms. Upon furtherprocessing, that is, during winding, these adhesions cause cracks on thesurface of the material, thereby giving rise to surface defects. As aconsequence, of course, adhesions are highly undesirable on annealednon-ferrous metal objects.

In this context, the term non-ferrous metals refers to alloys whose maincomponents are copper, tin, aluminum and lead, whereby many othercomponents are also possible such as, for instance, magnesium, nickel,etc.

In order to avoid adhesions when annealing steel strips, DE 4207394discloses the approach of changing the water-gas equilibrium in atargeted manner in the presence of H₂, CO₂, CO and H₂ O in theprotective-gas atmosphere in such a way that a totally oxidizingatmosphere is available at the end of the holding phase, while a totallyreducing atmosphere is available in the cooling phase. This approach,however, cannot be used for non-ferrous metals due to the much lowertemperatures that are found at times, for instance, 400° C. [752° F.],and due to the detrimental effect on the part of the reaction products,such as CO and H₂ O, on the oxidation mechanism.

SUMMARY OF INVENTION

The invention is based on the objective of creating a process with whichit is possible to avoid adhesions during the annealing of non-ferrousmetal objects, especially non-ferrous metal coils, in bell-typefurnaces.

In accordance with the invention the material being annealed is exposedto an inert or oxidizing protective-gas atmosphere. As a result, a thinlayer is formed during this time on the surface of the material beingannealed and/or an oxide layer that was previously present wasmaintained thus preventing the non-ferrous metal objects from adheringtogether.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in greater detail below with referenceto the example of an embodiment in conjunction with a drawing, whereby

FIG. 1 shows a state diagram of copper-tin;

FIG. 2 shows the course of the temperature over time and the compositionof a protective-gas atmosphere used for the adhesion-free treatment ofbronze wire.

DETAILED DESCRIPTION

First of all, the example of a copper-tin alloy (bronze) is employed toillustrate the problem of the adhesion of individual windings during theannealing of non-ferrous metal coils.

Due to their large solidification interval, copper-tin alloys tend toform zone crystals when they are cast. These solidification intervalsare one of the reasons for the reverse block segregation associated withpronounced differences in concentration throughout the cross section.This can be accompanied by exudation on the surface. These concentrationdifferences are the reason why a heterogeneous structure can alreadyform at low tin contents in the cast state. The extent of segregationdepends on the cooling conditions. The faster the cooling takes place,the lower the tin contents are at which the theoretical boundary of thehomogeneous area lies.

FIG. 1 depicts a state diagram of the copper-tin alloys. As can be seenin the illustration, the γ-crystal breaks down eutectically at 520° C.[968° F.] into α+δ, and the δ-phase, in turn, eutectically converts intoα+ε at a temperature of approximately 350° C. [662° F.], a process inwhich the compound Cu₃₁ Sn₈ is the δ-phase and Cu₃ Sn is the ε-phase.This conversion takes place extraordinarily slowly, so that technicalalloys display an (α+δ)-eutectoid in their final state, even when theyhave been cooled off slowly.

The tin-concentration differences within a crystal can amount to up to10%. Homogenization annealing has the objective of compensating forthese differences to the greatest extent possible. Dissolution of theδ-component during annealing is achieved within the range from 650° C.to 700° C. [1202° F. to 1292° F.], as a result of which there is aconsiderable increase in the dilatation. The yield strength also riseswhen the dilatation increases.

Copper-tin alloys are normally cast in an air atmosphere and are usuallycold-shaped. This means that the surface is strongly oxidized. For thisreason, strongly reducing protective-gas atmospheres are currentlyemployed during the homogenization annealing that has to be carried outsubsequently. The hydrogen fraction of commonly employed protectivegases amounts to approximately 100 vol-%. In this manner, the oxides arealready reduced in the heating phase. As a result of the reduction ofthe oxides, which is accompanied by exudation on the surface, thesurfaces of wires or strips are bright after the annealing treatment,although they adhere strongly. Further processing presupposes amechanical re-working of the surface, as a result of which it is verytime-consuming and costly.

Over the course of several laboratory experiments under operatingparameters, cold-shaped cast samples of bronze wire underwenthomogenizing annealing, first under reducing protective-gas atmospheres(75% N₂, 25% H₂). In the case of bronze wire, strong exudation wasobserved on the surface of the treated samples. This exudationfrequently tended to occur at those sites where the concentration of tindue to segregation was the highest. The strongly reducing protective-gasatmosphere obviously could not promote this process. Under reducing,hydrogenous atmospheres, reduction of the oxides on the surface alreadytakes place in parallel during the heating phase and this reduction ismost intense on the grain boundaries, which is comparable to a thermaletching.

The grain boundaries, which are open towards the outside, probably arethe sites at which a low-melting tin phase exuded as a result of aconversion of a structure that is not yet homogeneous. Since thewindings lie very closely together when coils are annealed, this givesrise to bridges, referred to as adhesions, which form a fused bondbetween two adjacent surfaces.

In additional experiments, the hydrogen fraction in the protective-gasatmosphere was constantly lowered. It was observed that, as thereduction capacity of the protective-gas atmosphere dropped, theexudation became increasingly less. Finally, experiments were conductedwith inert or oxidizing protective-gas atmospheres, whereby carbondioxide was employed as the oxidant.

An example of such a treatment is explained in FIG. 2. A gas mixturewith 15% vol-% CO₂ (rest N₂) ensured that the oxide layers weremaintained in the heating and holding phases and, depending on the alloyelements--for instance, during intermediate annealing of shaped wireswhich had already been cold-shaped--brought about an additionaloxidation by the CO₂ fraction, even at temperatures of 400° C. [752°F.]. In this manner, it was possible to stop exudation from the annealedmaterial by means of a protective sheathing of the surface with a thinoxide layer, and the batches were annealed without adhesions. At thesame time, better conditions were created for the homogenizationprocess.

Finally, in order to once again reduce the oxide layer maintained orre-constituted in the heating phase or at the beginning of the holdingphase, the N₂ --CO₂ protective-gas atmosphere was replaced by a purehydrogen atmosphere at the end of the holding phase. This createdstrongly reducing conditions for the end of the holding phase as well asfor the cooling period, and the oxide layers which protect againstadhesions were then eliminated. The batches were bright-annealed withoutthe occurrence of adhesions. The change over time of the decisiveparameters is presented once again in the overview below:

    ______________________________________                                        Test parameters for bronze wire treatment                                         Time [hours]                                                                            Temperature    CO.sub.2 [%]                                                                         H.sub.2 [%]                               ______________________________________                                        0          25° C. [77° F.]                                                               15       0                                             1 290° C. [554° F.]  15 0                                       2 530° C. [986° F.]  15 0                                       3 700° C. [1292° F.] 15 0                                       4 700° C. [1292° F.] 15 0                                       5 700° C. [1292° F.] 15 0                                       6 700° C. [1292° F.] 15 0                                       7 700° C. [1292° F.] 15 0                                       8 700° C. [1292° F.] 15 0                                       9 700° C. [1292° F.]  0 100                                     10  700° C. [1292° F.]  0 100                                   11  500° C. [932° F.]   0 100                                 ______________________________________                                    

Additional experiments have shown that the same results are obtainedwith other non-ferrous metal alloys such as, for instance, nickel brass(Cu--Ni--Zn).

The heat treatment of wire cast in air, which already has a pronouncedoxide layer, would also be possible in the holding phase, even with aninert protective-gas atmosphere, for example, with pure nitrogen. Theoxide layer could be subsequently reduced with hydrogen in the coolingphase in order to achieve a bright annealing result.

Individually as well as in any desired combination, the inventionfeatures disclosed in the preceding description, in the drawing as wellas in the claims can be essential for the realization of the inventionin its various embodiments.

I claim:
 1. Process to avoid adhesions during annealing of metal objectsconsisting of a non-ferrous metal alloy, comprising a heating, a holdingand a cooling phase, whereby the surface of the metal objects has anoxide layer, characterized in that during the holding phase the metalobjects are exposed to an inert protective-gas atmosphere, as a resultof which the oxide layer is maintained on the surface of the metalobjects being annealed thus preventing the non-ferrous metal objectsfrom adhering together, whereby at one of the end of the holding phaseand the beginning of the cooling phase, the inert protective-gasatmosphere is replaced by a reducing atmosphere of pure hydrogen, thusreducing the oxide layer and ensuring a bright surface of the annealedmetal objects.
 2. Process according to claim 1, characterized in thatthe inert protective-gas atmosphere consists of N₂.
 3. Process to avoidadhesions during annealing of metal objects consisting of a non-ferrousmetal alloy, comprising a heating, a holding and a cooling phase,characterized in that during the heating and during the holding phasethe metal objects are exposed to an oxidizing protective-gas atmosphere,as a result of which a thin oxide layer is formed on the surface of themetal objects being annealed thus preventing the non-ferrous metalobjects from adhering together, whereby at one of the end of the holdingphase and the beginning of the cooling phase, the oxidizingprotective-gas atmosphere is replaced by a reducing atmosphere of purehydrogen, thus reducing the oxide layer and ensuring a bright surface ofthe annealed metal objects.
 4. Process according to claim 3,characterized in that the oxidizing protective-gas atmosphere containscarbon dioxide.
 5. Process according to claim 4, characterized in thatthe oxidizing protective-gas atmosphere contains at least 10 vol-% ofcarbon dioxide.